Motoya Nakamura/The OregonianA Japanese flag flies above wreckage in front of the city hospital in Onagawa, a community devastated by the March 11 tsunami and 9-magnitude earthquake. Experts estimate that at least 5,000 Oregonians will die in a similar quake and tsunami here. The only question, they say, is when.

Experts armed with seabed core samples and findings from Japan are ready to place odds on the likelihood of a giant earthquake rocking the Northwest.

Within the next 50 years, they say, Washington and northern Oregon face a 10 to 15 percent probability of an offshore quake powerful enough to kill thousands and launch a tsunami that would level coastal cities. Off southern Oregon, the probability of an 8-or-higher magnitude earthquake is greater -- 37 percent, according to Oregon State University's Chris Goldfinger, one of the world's top experts on subduction-zone quakes.

Goldfinger and other authorities who spoke at a Portland conference this week say the Northwest is dangerously unprepared for a massive quake they consider inevitable at some point. At least 300,000 Oregon children attend school in buildings vulnerable to collapse when the Big One comes.

"I think every parent should know this," said Kit Miyamoto, an earthquake engineer from Japan whose company is helping repair quake-damaged structures in Haiti. "Those schools should be banned."

Earthquake experts are speaking with new urgency after the Japanese earthquake and tsunami that killed more than 24,000 March 11, shattering long-held assumptions on safety and survival. A much smaller New Zealand quake in February showed what can happen in a city similar to Portland, killing 181 and destroying thousands of houses in Christchurch.

Researchers say giant tectonic plates off Oregon's coast are locked in a slow-motion collision, accumulating energy that will ultimately be released when sections of Earth's crust slip. An earthquake in the Cascadia subduction zone could launch a tsunami resembling Japan's towering tidal wave, flattening Seaside and other low-lying cities.

Oregon is not nearly as prepared as Japan for a major earthquake, let alone a tsunami, said experts at the meeting held Friday in Portland State University's seismically reinforced Lincoln Hall. Oregon stores much of its liquid fuel, for example, in tanks on soil prone to liquefaction along the Willamette River north of Portland.

Japan built sea walls and other elaborate fortifications to withstand a tsunami from an 8.2-magnitude quake, the maximum officials expected along the country's northeast coast. But one scientist, Yasutaka Ikeda of Tokyo University, predicted 10 years ago that a 9-magnitude earthquake would shake the region.

Goldfinger and his colleagues have advanced the new field of paleoseismology, examining sediments and rocks for signs of ancient earthquakes. They took core samples recently in Japan's Sendai plain, confirming that the Jogan tsunami in the year 869 swept farther than the March 11 wave, stopped only by hills 3 miles inland in what is now Sendai city.

Goldfinger's team has used similar techniques to date and fingerprint 19 undersea ruptures that have occurred during the last 10,000 years along the entire tectonic-plate margin from California to British Columbia.

Twenty-two more earthquakes have occurred along the southern parts of the Cascadia zone off Oregon, making such massive ruptures rather routine geologic events. Cascadia quakes seem to trigger activity along the San Andreas fault line to the south.

The largest Cascadia quake, known as T11, hit 5,900 years ago. The most recent one, called T1, occurred around 9 p.m. Jan. 26, 1700. The roughly 9-magnitude quake sent a tsunami to Japan.

That was 311 years ago, giving the Juan de Fuca and North American plates more than three centuries to strain against each other off Oregon, amassing energy for release. The farther south along the Northwest coast, the more frequently quakes occur.

An 8.5-magnitude earthquake would kill an estimated 5,000 Oregon residents at minimum, said Yumei Wang, geohazards team leader at the Oregon Department of Geology and Mineral Industries.

Many would probably die in unreinforced masonry structures prone to collapse, she said, including school buildings listed on her agency's Web site. State grants to upgrade schools and emergency buildings have lapsed.

Many Oregon bridges are also vulnerable, said Peter Dusicka, a PSU civil engineering professor. His modeling projects Oregon bridge damage exceeding $1 billion from a 9-magnitude Cascadia quake. A less powerful 6.5-magnitude earthquake in the Portland hills would cause more than $1.5 billion in bridge damage, Dusicka expects.

"It wasn't really until the 1990s that a worthwhile earthquake load was considered for bridges" in Oregon, Dusicka said. "Yet most of the building was in the '60s and '70s."

Engineers have learned from over the years from each major earthquake, Dusicka said, finding that seismic retrofitting can reduce bridge damage. Portland's Marquam Bridge, built in 1966, had a basic retrofit in 1995 with cables tying together the approaches to prevent a super-structure failure. But its vulnerable columns have not been addressed.

Other spans, such as the Fremont Bridge, built in 1973, remain vulnerable. "In Oregon we have done next to nothing," Dusicka said. The state has no current funding for seismic bridge retrofits, he said.

Likewise Oregon has no seismic requirements for dams, and no mandate to evaluate dams and mitigate risks, Wang said.

Engineers doing seismic retrofits often concentrate on preventing structures from collapsing and killing people, but not on enabling them to survive shaking well enough to be used after an earthquake, Miyamoto said. The result is "throw-away" buildings that may need to be demolished.

In Christchurch, for example, downtown high-rises that avoided collapse but are now leaning dangerously must be demolished, Miyamoto said, meaning that large sections of the city will be out of commission for months. Downtown Portland would have similar problems, he said.

Scientists have found in California and elsewhere that that it tends to take several earthquakes before politicians devote money to preparation. Fixing everything isn't possible or affordable, so it's best to set priorities within a defined budget, Miyamoto said.

Blake Patsy, managing principal at KPFF Consulting Engineers, said substandard buildings can often be upgraded for a reasonable cost and perform significantly better. Engineers learned from the 1971 San Fernando earthquake that tilt-up buildings used for warehouses and school gymnasiums are especially vulnerable to collapse.

Shelly Duquette, of the Structural Engineers Association of Oregon, is developing a plan to mobilize structural engineers after an earthquake to assess building damage to people will know whether they are safe to use.

But such preparations are the exception.

Wang attended a tsunami drill in Pacific City a few weeks ago, before departing on a fact-finding trip to Japan. Unlike in a real tsunami, which could occur in the middle of the night without notice, residents were told in advance. They practiced evacuating on a sunny day.

"And in the perfect conditions," Wang said, "they didn't all make it out."